What I'm after is to keep motor rumble travelling --- conduct it, punch it's ticket, make sure it doesn't leave any baggage behind at the station--- and send it on it's way.

If it manages the whole trip and exits at the spikes, cones, or the Aurios I'm now using, great. Sayonara, keep moving. If it falls asleep on the trip and kind of just zones out and deteriorates to nothing on it's way thru the plinth..... Fine, as long as it doesn't wake up again.

What I'm not after is having the motor rumble bouncing back off a weird boundary layer or pinging off steel fasteners and coming back in a time & phase shifted way to the armboard. In my previous plinth I ended up using hardwood pegs to attach the armboard for a similar reason. Steel (I tried it) conducted some aspect of audio-band motor rumble directly to the armboard it was fastening.

What I'm also not after is any detour or translation taking place with the motor signature, hence no 'contrast' layers for me in the plinth -- whether similar to the birch ply or dissimilar, like lead. The glue I'm using, which luthiers use for difficult bonding chores like violin bodies, does more to integrate adjacent layers, and fuse them physically and sonically ...than it does to form any intermediary layer.

The 'purist' (take w/ grain of salt, everybody's a purist, and nobody agrees) approach I'm taking on my current project is : Baltic Birch only, hardrock maple armboard, hide glue only, roller bearings for footers. Wide and deep (20x24"), but not too (3.75") thick, keeping that heavy motor very low to the ground, and very seriously rigid.

Every plinth maker seems to want the 'Only Way To Do It' crown for his way, which is ridiculous. I will, however, mention the following :

Nine out of ten articles about 301/401 practices properly mention that the Japanese are roundly regarded as the sponsors of the Garrard 301 Rennaisance that took place somewhere around the mid-eighties, just as we here in the enlightened West were discovering Compact Disc to our extended delight. 

If you take a long trip thru the Japanese sites that show a 301/401 being used in Japan, most are in plinths of stacked ply & glue. Coincidence ? Possibly. Can it be done other ways ? Very definitely could be, but isn't. Conclusion ? Your call.

One last thing to wonder about is why Shindo-San doesn't do birchply plinths, when in fact he knows quite a lot about the issue. I think the answer there has to do with the fact that he's in the business of selling $20K super-luxe iterations of the 301, and can afford, in those budget confines, to be more luxurious with every phase of design and materials.
( A parallel question would be --- can you order a top-of-the-line Lexus or Benz without leather seats ? I don't think you can. A certain level of clientele don't do fabric seats, don't ever take the subway, and don't do birch ply. )
That's my guess, anyway.